Don Keele & the Constant-Directivity CRT36 Speaker

Speaker engineer Don Keele almost wrote the book on measuring speakers. so when I bumped into him at RMAF and he told me he had a new speaker on show, I went straight to his room. There, I saw and heard the CBT36, which as you can see, is very unusual in both appearance and design. CBT stands for Constant Beamwidth Transducer and is based on unclassified military underwater sound beam-forming research. There are 72 0.75" tweeters crossed over to 18 3.5" Dayton Audio midrange units, arranged in groups that, with the 36° arc of the 5'-tall array, gives sound that doesn't change its balance as the listener sits or stands. And as you can see from the mirror that Don has slipped in front of the speaker in my photo, the ground-plane reflection of the array effectively doubles its height.

The CBT36 is available now from Parts Express as a kit for a very reasonable $1980/pair or as fully finished speakers in the future from Audio Artistry for $8500/pair. Demmed with Jeff Rowland Design Group amplification and crossed over to an active woofer below 60Hz with a DEQX DSP unit, these speakers produced one of my best sounds at the 2011 RMAF.

I looked at the relevant page on Parts-Express, but I still can't figure out whether the kit includes any of the cabinetry. If it consists of just the drivers and crossover, getting that (presumably critical) cabinet shape right in a home workshop would not be an easy task, it seems to me.

At RMAF, I went to the Parts Express booth and they had the unfinished cabinet on display. They confirmed that the kit includes all the drivers and cabinet. I'm not sure about the crossover since the Audio Artistry demo was using an active crossover and bi-amping the speakers.

I was impressed with the demo. The soundstage was huge, and the speaker seemed to work as advertised with a very even response throughout the room. It does not image like a monitor, but I thought it was very balanced.

I talked to Don about the theory behind the design and it makes sense when he explains it. He had some very interesting graphs from his paper showing the frequency distribution based on the arc shape with graded attenuation on the drivers higher up on the arc.

My understanding is that the crossovers are not included in the price, and you have to purchase either the DEQX or the Behringer digital crossovers. AA then provides the correct crossover settings. The price also doesn't include subs, which are pretty much mandatory for full range operation. So it is a pretty complex setup and the costs will add up even though on first glance it looks very inexpensive. When I heard the system on Friday, it sounded way too bright and the subs were turned up too high. I'm assuming they tweaked it later for better balance. As for the kit, there's going to be a lot of wiring involved with all those drivers, though the tweeters come in clusters of 4 on a circuit board. Still, this is not a project I would suggest for anyone other than a very experienced DIY loudspeaker builder.

Yes, this is not a novice's kit. From inception, this is an active speaker (which is a good thing) so a quality active crossover and the understanding of how to use it are necessities.

I would avoid using the Behringer as your crossover unless you already have one on hand or your money is really tight. Also, if you are thinking about going the insane route of getting a modded Behringer, let me suggest that you are better off spending your money on a Xilica digital XO than throwing money at a piece of dung like the DCX2496. It's not just the converters that make the sound of the Xilica. It's filter algorithms are top notch (as they should be with a pro sound unit). They make several variants with the top-line XD models starting at $1,775 (proaudiosolutions.com). The ideal solution, though, would be a room correction unit like the DEQX that also handles the crossover duties.

As an aside, Xilica designs pro loudspeaker management & DSP units for other companies and I believe the XD processors are used by Legacy Audio for their Whisper XD loudspeaker.

As to ceiling height, it's really not any more of an issue with the CBT as it is with any other speaker since the array projects most of it's sound forward. The curved array and Legendre Shading (the speakers output is rolled off the further you move from the floor) do three very important things: the sound won't change as you go from seated to standing, the level doesn't change very much as you move around the room and distortion is very low because of the number of drivers used. Also, due to the very small driver diameter, the side to side radiation pattern is almost a perfect hemisphere. What you have is a very high dynamic range speaker system with a very uniform, wide radiation pattern up to 10khz.

One more thing. The cabinet shown in the photo is metal, not the wood cabinet that Parts Express will be offering. ALso, Dr. Don is a very nice guy who's about as nutty as he looks in the photo!

I'd seen close-up photos of an earlier but very similar looking CBT-36 that had a pillar in the rear and appeared to be welded to the base rather than attached with a bracket as in this photo. I also saw a couple photos from the shop showing a very different wooden cabinet with the prototype in the background. Or, may be, I was under the undue influence of solvent fumes and cheap beer that day.

Anywho... if the actual cabs look like the ones in the photo and at the AA web site, that would be awesome! Great job Dr. Don & Marshall K.

One unimportant question: Are the tweets 1/2 or 3/4 inchers? I've just seen 2 different specs. One more: can you get black anodized tweeters or do the only come in shiny aluminum as pictured. The black looks cooler.

The tweeter cone diameter is 0.5 inch but the overall tweeter dimension is 0.72 inch.

We ran samples of a black cone tweeter and built a prototype but almost 100% of those that compared the two speakers preferred the silver cone cosmetically. Long term, both options may be offered but it will depend on how many requests we get for the black cone.